When flying a helicopter, the pilot needs to monitor numerous instruments on the instrument panel, most of which instruments represent the operation of the engine unit and of the aircraft. For physical reasons, there are numerous limits that the pilot must take into account at all times while flying. These various limits depend in general on the stage of flight and on outside conditions.
Most presently-built helicopters are fitted with one or two free-turbine engines. Power is then taken from a low pressure stage of the free turbine, which stage is mechanically independent of the compressor unit and of the high pressure stage of the engine. The free turbine of such an engine rotates at a speed lying in the range 20,000 revolutions per minute (rpm) to 50,000 rpm, so a reduction gearbox is needed for its connection to the main rotor which rotates at a speed that lies substantially in the range 200 rpm to 400 rpm: this is the main transmission gearbox.
The thermal limits of the engine and the torque limits of the main transmission gearbox serve to define three normal operation ratings for a gas turbine engine:                takeoff rating, usable for five to ten minutes and corresponding to the transmission gearbox being subjected to a level of torque and the engine to a degree of heating that can be accepted for a short time without significant damage: this is maximum takeoff power (PMD);        maximum continuous rating during which, at all times, care is taken to avoid exceeding the capabilities of the transmission gearbox, or the maximum heating that can be accepted continuously by the high pressure blades of the first stage of the turbine: this is maximum continuous power (PMC); and        maximum transient rating, optionally capped by regulations: this is referred to as maximum transient power (PMT).        
There also exist emergency overload ratings on multiengine aircraft, that are used in the event of one engine breaking down:                emergency rating during which the capabilities of the inlet stages of the transmission gearbox and the temperature margin of the engine are used to the full: this is referred to as super-emergency power (PSU) and can be used at most for thirty consecutive seconds, and three times only in any one flight; if PSU has been used, then the engine must be removed and overhauled;        emergency rating during which the capabilities of the inlet stages of the transmission gearbox and the temperature margin of the engine are used to a considerable extent: this is referred to as maximum emergency power (PMU) and can be used for two minutes after SEP or for two minutes and thirty seconds consecutively at the most; and        emergency rating during which the capabilities of the inlet stages of the transmission gearbox and the temperature margin of the engine are used, but without inflicting damage: this is referred to as intermediate emergency power (PIU) and can be used for thirty minutes or continuously for the remainder of the flight after an engine breakdown.        
The engine manufacturer uses calculation or testing to draw up curves repenting the power available from a gas turbine engine as a function of altitude and temperature, and for each of the above-defined ratings.
The limits specified are generally monitored via three parameters: gas generator speed; engine torque; and the temperature at which gas is ejected into the inlet of the free turbine, respectively written Ng, Cm, and T4 by the person skilled in the art.
Document FR 2 749 545 discloses a pilot indicator that identifies that one of the monitoring parameters of the engine that is closest to its limit. The information relating to the limits that need to be complied with is thus reduced to a single display, firstly making it possible to provide a summary by presenting solely the result of this summary in order to simplify the task of the pilot, and secondly making it possible to save space in the instrument panel. This produces a “limiting parameter” selected from said monitoring parameters of the engine, and constituted by the value of the monitoring parameter which is presently the closest to its limit value. That is why such an indicator is also referred to below as a first limit instrument or in abbreviation FLI.
This FLI thus makes it possible to determine the present value at any given instant of the limiting parameter. However, when about to perform a maneuver, the pilot cannot tell whether the limiting parameter is going to exceed its limit.